1. Field of the Invention
The invention relates to improvements in the preparation of catalysts for shape-selective reactive conversions of aromatic hydrocarbon compounds. More specifically, the invention relates to improvements in methods of modifying zeolite catalysts and shape-selective hydrocarbon conversion processes using such catalysts.
2. Description of the Prior Art
The term shape-selective catalysis describes unexpected catalytic selectivities in zeolites. The principles behind shape selective catalysis have been reviewed extensively, e.g., by Chen N Y, Garwood W E, and Dwyer F G, Shape Selective Catalysis in Industrial Applications, Marcel Dekker, Inc., New York (1989). Within a zeolite pore, hydrocarbon conversion reactions such as paraffin isomerization, olefin skeletal or double bond isomerization, oligomerization, and aromatic disproportionation, alkylation, or transalkylation reactions are governed by constraints imposed by the channel size. Reactant selectivity occurs when a fraction of the feedstock is too large to enter the zeolite pores to react. Product selectivity occurs when some of the products cannot leave the zeolite channels. Product distributions can also be altered by transition state selectivity in which certain reactions cannot occur because the reaction transition state is too large to form within the zeolite pores or cages. Another type of selectivity results from configurational diffusion where the dimensions of the molecule approach that of the zeolite pore system. A small change in dimensions of the molecule or the zeolite pore can result in large diffusion changes leading to different product distributions. This type of shape selective catalysis is demonstrated, for example, in selective disproportionation of toluene to para-xylene.
Para-xylene is a very valuable commercial product useful in the production of polyester fibers. The catalytic production of p-xylene has received much attention in the scientific community and various methods for increasing catalyst para-selectivity have been described.
The synthesis of p-xylene is typically performed by methylation of toluene over a catalyst under conversion conditions. Examples are the reaction of toluene with methanol as described by Chen et al., J. Amer. Chem. Soc. 101:6783 (1979) and toluene disproportionation, as described by Pines in The Chemistry of Catalytic Hydrocarbon Conversions, Academic Press, New York, p. 72 (1981). Such methods typically result in the production of a mixture including p-xylene, o-xylene, and m-xylene. Depending upon the para-selectivity of the catalyst and the reaction conditions, different percentages of p-xylene are obtained. The yield, i.e., the amount of feedstock actually converted to xylene, is also affected by the catalyst and the reaction conditions.
The equilibrium reaction for the disproportionation conversion of toluene to xylene and benzene yields about 59% mixed xylenes and benzene, with the balance being toluene. Of the converted product, about 57.6 wt % is mixed xylenes, with the remainder being benzene. The equilibrium distribution of the various xylenes in the mixed xylenes fractions is about 24% para-xylene, about 54% meta-xylene, and about 22% ortho-xylene. Given the limitations in the conversion rate and selectivity of this reaction, the total p-xylene yield (-xylene purity) is only about 8.2%. The unique importance of p-xylene motivates the search for methods of improving conversion and selectivity of this and related reactions.
The para-selectivity of hydrocarbon conversions can be improved by modifying the processing qualities of zeolite catalysts. One method by which the para-selectivity of such catalysts can be improved is by modifying the catalyst through treatment with xe2x80x9cselectivating agents.xe2x80x9d Modification methods have been suggested wherein the catalyst is modified by treatment prior to use to provide a silica coating. For example, U.S. Pat. Nos. 4,477,583 and 4,127,616 disclose methods wherein a catalyst is contacted at ambient conditions with a modifying compound such as phenylmethyl silicone in a hydrocarbon solvent or an aqueous emulsion, followed by calcination. Such modification procedures have been successful in obtaining para-selectivity of greater than about 90%, but with commercially unacceptable toluene conversions of only about 10%, resulting in a yield of not greater than about 9%, i.e., 10%xc3x9790%. Such processes also produce significant quantities of o-xylene and m-xylene, thereby necessitating expensive separation processes to separate the p-xylene from the other isomers.
Typical separation procedures include costly fractional crystallization and adsorptive separation of p-xylene from other xylene isomers which are customarily recycled. Xylene isomerization units are then required for additional conversion of the recycled xylene isomers into an equilibrium xylene mixture comprising p-xylene. Those persons who are skilled in the art appreciate that the expense of the separation process is proportional to the degree of separation required. Therefore, significant cost savings are achieved by increasing selectivity to the para-isomer while maintaining commercially acceptable conversion levels.
It is, therefore, highly desirable to provide a regioselective process for the production of p-xylene from toluene while maintaining commercially acceptable toluene conversion levels. But it is also highly desirable to provide processes that are regioselective, or at least highly selective for the production of other types of products whose synthesis can be catalyzed by zeolite-type catalysts. One notable such conversion process is the related ethylbenzene conversion in a feed of ethylbenzene and mixed m- and o-xylenes.
In view of the above considerations, it is clear that existing catalysts and processes for shape selective hydrocarbon conversion are critical to improving the quality and yield of materials suitable for commercial manufacturing. Accordingly, it is one of the purposes of this invention to overcome the above limitations in shape selective hydrocarbon conversion processing, by providing a catalyst for shape selective hydrocarbon conversion processes wherein the catalyst has substantially stabilized hydrogenation-dehydrogenation functionality. The catalyst enables the artisan to perform shape-selective hydrocarbon conversion processes with high product yields and low by-product yields. In particular the catalyst improves the yields and purities of products from aromatics conversion processes such as disproportionations, isomerizations, alkylations, and transalkylations.
It has now been discovered that these and other objectives can be achieved by the present invention, which provides a catalyst having hydrogenation-dehydrogenation functionality and a method for preparing such a catalyst.
In one embodiment, the invention is a catalyst for use in shape-selective hydrocarbon conversion processes, wherein the catalyst has been modified according to a procedure comprising the steps of:
(a) permeating a hydrogenation-dehydrogenation functional metal into a catalyst to provide a metal-modified catalyst, and
(b) gradient selectivating the metal-modified catalyst under an in situ selectivation protocol that comprises a progressive temperature gradient to provide a functionalized catalyst,
wherein product yield and distribution from a hydrocarbon conversion over the functionalized catalyst is substantially stable with increasing time on stream.
Preferably the catalyst has been modified by permeating into the catalyst a functional metal selected from the group consisting of Groups 3 to 15 of the Periodic Table. More preferably, the functional metal is selected from the group consisting of cadmium, cobalt, copper, gold, iron, mercury, molybdenum, nickel, osmium, palladium, platinum, rhenium, rhodium, ruthenium, silver, zinc, and mixtures thereof.
The catalyst is preferably modified by an in situ selectivation protocol in which the gradient selectivating comprises a stepped or continuous temperature gradient. The progressive temperature gradient can include a lower temperature bound of from about 350xc2x0 C. to about 460xc2x0 C., and an upper temperature bound of from about 420xc2x0 C. to about 550xc2x0 C. Preferably, the lower temperature bound is from about 380xc2x0 C. to about 430xc2x0 C., and the upper temperature bound is from about 450xc2x0 C. to about 510xc2x0 C. Moreover, the in situ selectivation protocol preferably further comprises a pressure of from above 0 psig to about 2000 psig, a H2/HC mole ratio of from 0 to about 20, and a WHSV of from about 0.1 hrxe2x88x921 to about 100 hrxe2x88x921.
The catalyst can also have been further modified by preselectivating the catalyst, i.e., by exposing the catalyst to at least one preselectivation sequence, each preselectivation sequence comprising contacting the catalyst with an organosilicon compound and then calcining the contacted catalyst.
The modified catalyst is preferably a catalyst that comprises a zeolite selected from the group consisting of ZSM-5, ZSM-11, ZSM-12, ZSM-21, ZSM-22, ZSM-23, ZSM-35, ZSM-38, ZSM-48, ZSM-57, and ZSM-58.
In another embodiment, the invention is a method of modifying a catalyst for use in shape-selective hydrocarbon conversion processes, comprising:
(a) permeating a catalytic amount of a hydrogenation-dehydrogenation functional metal into a catalyst to provide a metal-modified catalyst, and
(b) gradient selectivating the metal-modified catalyst under an in situ selectivation protocol that comprises a progressive temperature gradient to provide a functionalized catalyst,
whereby product yield and distribution from a hydrocarbon conversion over the functionalized catalyst is substantially stable with increasing time on stream.
In still a further embodiment, the invention is a process for shape-selective hydrocarbon conversion over a zeolite catalyst, comprising converting a hydrocarbon under conversion conditions over a functionalized catalyst modified according to a method comprising the steps of:
(a) permeating a catalytic amount of a hydrogenation-dehydrogenation functional metal into a catalyst to provide a metal-modified catalyst, and
(b) gradient selectivating the metal-modified catalyst under an in situ selectivation protocol that comprises a progressive temperature gradient to provide a functionalized catalyst,
whereby product yield and distribution from the hydrocarbon conversion over the functionalized catalyst is render substantially stable with increasing time on stream.
The conversion conditions for the hydrocarbon conversion process preferably comprise a temperature of from about 100xc2x0 C. to about 760xc2x0 C., a pressure of from above 0 psig to about 3000 psig, a H2/HC mole ratio of from 0 to about 100, and a WHSV of from about 0.08 hrxe2x88x921 to about 2000 hrxe2x88x921.
These and other advantages of the present invention will be appreciated from the detailed description and examples which are set forth herein. The detailed description and examples enhance the understanding of the invention, but are not intended to limit the scope of the invention.